Abstract <p>Cheese whey is one of the most abundant by-products of the dairy industry and represents a serious environmental challenge due to its high organic load and disposal costs. This study explored a sustainable biotechnological approach for whey valorization using <i>Trichoderma harzianum</i> as a fermentative agent. The fungus was able to grow both in solid and liquid whey-based media, generating a bioferment with modified physicochemical properties, including reduced sugar content and altered protein and pH profiles. Application of the bioferment at different concentrations was evaluated on seed germination of <i>Raphanus sativus</i> and <i>Solanum lycopersicum.</i> Results showed that low concentrations (5–10%) promoted seed germination and early seedling development, while higher concentrations (&gt; 15%) were inhibitory. The novelty of this work lies in demonstrating that biologically treated whey can be transformed into a functional agricultural input with bio-stimulant activity, thus integrating a circular bioeconomy model that reduces dairy waste pollution while producing a value-added product for horticultural production.</p> Statement of Novelty <p>This study demonstrates that cheese whey, a highly polluting dairy by-product, can be biotransformed by<i> Trichoderma harzianum</i> into a bioferment with plant biostimulant activity. The novelty lies in the dual outcome of wastewater bioremediation and the generation of a value-added agricultural input. The impact of this approach is the integration of a circular bioeconomy model that simultaneously reduces environmental pollution, lowers disposal costs, and enhances tomato seed germination, offering a sustainable and replicable solution for dairy and horticultural sectors.</p> Graphical Abstract <p></p> Highlights <p><UnorderedList Mark="Bullet"> <ItemContent> <p>Cheese whey was biologically valorized using Trichoderma harzianum.</p> </ItemContent> <ItemContent> <p>Fungal growth generated a bioferment with modified physicochemical properties.</p> </ItemContent> <ItemContent> <p>Bioferment at 5–10% enhanced tomato seed germination and seedling vigor.</p> </ItemContent> <ItemContent> <p>High concentrations (&gt;15%) showed inhibitory effects on plant development.</p> </ItemContent> <ItemContent> <p>Novel approach integrates whey bioremediation with agricultural bio-stimulant use.</p> </ItemContent> </UnorderedList></p>